1. Field of the Invention
The present invention relates to a search coil assembly for an inductive search device for detecting objects of electrically conductive material hidden from direct view as, for example, when buried in the soil or they are surrounded by other materials having a lower electrical conductivity.
2. Description of Related Art
Inductive search devices, whether they are designed as mine search devices, or as so-called "treasure search devices", or as a stationary metal detector, have a high sensitivity, for the most part. Accordingly, if no special counter measures are taken, even the approach of electrically non-conductive or low-conductive objects will effect an undesired and disturbing signal voltage. The reasons for this phenomenon are that the electrical field being built up in the area of the search coil assembly and the dielectric constant in the field area are modified by the approach of the mentioned objects. These capacitive currents will produce a disturbance of adjustment and, thus, interference signal voltages. The most widely used counter measure relies on preventing propagation of the electrical fields outside the search coil assembly search coil with an electrical shield biased to a reference potential, which in general is ground.
The shielding approach is effected, in conventional search coil assemblies, by wrapping the shield around the search coil, as described in DE-GM 83 36 122, for example, such that the latter will be completely surrounded by the shield. Such a shield can be metal foil, a net-type web of thin wires, or a single wire arranged in a tight layer on the search coil. Care must always to be taken, however, that the shield does not form a closed loop. Currents induced in a closed loop would have a retroactive effect on the search coil and would cause interference signals therein. Even the eddy currents generated in such a shield would produce interference signals in the search coil, which are particularly unfavourable, as they continuously vary under temperature change. Metal foils can, therefore, only be used with low frequencies or by providing an approximately large distance between shield and search coil. The same applies for net-type webs, unless they are made of insulated wires.
From GB-A1 22 05 408, there is known a search coil assembly, wherein the search coil is embedded in a block of synthetic plastic material. The block is covered by a closed metal shield, which is held, however, at a sufficient distance from the search coil to prevent eddy currents being formed in the shield. In the same document, another shield is described, which can, because of the reduced electrical conductivity of its material, even be located relatively close to the search coil. Specifically, the shield material contains carbon, e.g. graphite, which is applied in liquid condition for hardening on the inner surface of the search coil frame. Such a shield serves, in the present case, for removing electrostatic charges, and only has limited use in suppressing capacitive interference effects. The necessity of establishment of a satisfactorily high conductivity contact with the carbon-containing material poses problems.